GHG Mitigation Potential of Different Grazing Strategies in the United States Southern Great Plains

Transcription

1 Sustanablty 2015, 7, ; do: /su Artcle OPEN ACCESS sustanablty ISSN GHG Mtgaton Potental of Dfferent Grazng Strateges n the Unted States Southern Great Plans Tong Wang 1, *, W. Rchard Teague 2, Seong C. Park 2 and Stan Bevers 3 1 Department of Economcs, South Dakota State Unversty, Brookngs, SD 57007, USA 2 Texas A&M AgrLfe Research, Vernon, TX 76384, USA; E-Mals: rteague@ag.tamu.edu (W.R.T.); scpark@ag.tamu.edu (S.C.P.) 3 Texas A&M AgrLfe Extenson, Vernon, TX 76384, USA; E-Mal: s-bevers@tamu.edu * Author to whom correspondence should be addressed; E-Mal: tong.wang@sdstate.edu; Tel.: ; Fax: Academc Edtor: Marc A. Rosen Receved: 21 July 2015 / Accepted: 25 September 2015/ Publshed: 30 September 2015 Abstract: The possblty of reducng greenhouse gas (GHG) emssons by rumnants usng mproved grazng s nvestgated by estmatng GHG emssons for cow-calf farms under lght contnuous (LC), heavy contnuous (HC) and rotatonal grazng, also known as mult-paddock (MP), management strateges n Southern Great Plan (SGP) usng lfe cycle assessment (LCA). Our results ndcated a GHG emsson wth these grazng treatments of kg CO2e calf 1 year 1 for cow-calf farms n SGP regon, whch s hgh, compared to that for other regons, due to the hgh percentage (79.6%) of enterc CH4 emssons caused by relatvely lower feed qualty on the unfertlzed rangeland. Senstvty analyses on MP grazng strategy showed that an ncrease n grass qualty and dgestblty could potentally reduce GHG emsson by 30%. Despte hgher GHG emssons on a per calf bass, net GHG emssons n SGP regon are potentally negatve when carbon (C) sequestraton s taken nto account. Wth net C emsson rates of , and 89.5 kg C ha 1 year 1 after convertng from HC to MP, HC to LC and from LC to MP, our analyss ndcated cow-calf farms convertng from contnuous to MP grazng n SGP regon are lkely net carbon snks for decades. Keywords: rotatonal grazng; greenhouse gas; lfe cycle assessment; C sequestraton; Southern Great Plans

2 Sustanablty 2015, Introducton Rumnants, partcularly beef cattle, are perceved by many as a problem snce they are a source of greenhouse gas (GHG) due to the methane produced by rumen fermentaton [1]. However, t s premature to decde on approprate management actons or polces untl full ecosystem analyses have outlned net emssons by consderng all emssons compared to carbon sequestraton assocated wth dfferent optons n the beef producton chan [2]. Snce the major porton of the beef producton chan nvolves anmals grazng on perennal pastures, an mportant ntal step would be to gather GHG emssons and carbon (C) sequestraton data to determne net emssons usng lfe cycle assessment (LCA) for dfferent grazng strateges on perennal pastures. For beef cattle producton, management practces n dfferent regons vary greatly n terms of stockng rate, mean cow sze, calvng season, prmary forage types and fertlzer use [3]. Therefore, an deal LCA model s one that s regonally specfc. Our LCA modelng s applcable to the cow-calf only producton phase n the Southern Great Plans (SGP) regon of USA, where one thrd of US cow-calf only farms are located [4], and no known net-emsson LCA study regardng cow-calf producton on perennal pastures has been conducted. Tradtonal ranchng n the South Central U.S. has generally been based on contnuous yearlong grazng practces. Rotatonal grazng, also known as mult-paddock grazng (MP), has been recommended snce the md-20th century as an mportant tool to adaptvely manage grazng land ecosystems for the purpose of sustanng productvty and mprovng anmal management. Under rotatonal grazng management, one paddock s grazed at a tme whle the other paddocks recover. There s publshed and anecdotal evdence from producers that, f appled approprately to produce most advantageous results, rotatonal grazng can lead to mproved forage and lvestock producton [5 8]. Smulaton modelng also ndcates that there tend to be larger proft margns and restoraton of ecologcal condton wth rotatonal grazng compared to tradtonal grazng [9,10]. However, few studes have been conducted to compare GHG emssons and C sequestraton relatons among dfferent grazng strateges. In ths study we calculate the carbon footprnts for cow-calf farmers under contnuous and rotatonal grazng strateges usng lfe cycle assessment (LCA) modelng, whch s a standard assessment of the envronmental mpacts assocated wth a wde range of agrcultural systems usng a cradle-to-grave approach. Compared to the sector approach whch only ncludes emssons from drect farm actvtes, LCA also ncludes ndrect emssons generated by farm nputs and pre-chan actvtes. Prevous LCA studes on beef producton have consstently reported that the cow-calf phase contrbutes the most emssons to the overall beef producton system [11]. However, they generally omtted carbon sequestraton, whch has great potental to mtgate GHG emssons for cow-calf producton as C sequestraton exceeds emssons when anmals feed solely by grazng perennal pastures [5,12]. In addton, net emssons are rarely analyzed on the same farm to estmate the GHG balance [13] and changes n C stock resultng from dfferent grazng management practces are generally not known [14]. As grazng management practces have mpacts on both GHG emssons and carbon sequestraton, t s mportant to consder C sequestraton n conjuncton wth GHG emssons on the same ranch to provde an objectve evaluaton of the GHG mtgaton potental of advanced grazng management strateges.

3 Sustanablty 2015, In our study we consdered both GHG emssons and C sequestraton to calculate net GHG emssons for cow-calf farms under dfferent grazng strateges. Based on Teague et al. [15], three grazng management alternatves on neghborng commercal ranches n three proxmate countes n north Texas tall grass prare are consdered, ncludng: (1) contnuous grazng wth lght stockng (LC), representng the best-case scenaro for contnuous grazng; (2) tradtonal heavly stocked contnuous grazng (HC), representng the most commonly used grazng management; and (3) adaptvely managed and stocked rotatonal grazng, or mult-paddock grazng (MP), representng the best case scenaro for rotatonal grazng [6,8,10]. GHG emssons were evaluated for the cow-calf farms under the three dfferent grazng strateges usng LCA approach. In addton, sol organc carbon (SOC) stock under the same grazng strateges were calculated usng the sol carbon parameters measured by Teague et al. [15]. Based on both GHG emsson and SOC stock values, we developed net C emsson budgets for dfferent farm transton scenaros, namely transtng from HC to MP, from HC to LC and from LC to MP. 2. Methods and Materals 2.1. The Study System The goal of our study s to assess the GHG emssons and carbon sequestraton from dfferent grazng management optons for representatve cow-calf enterprses n the Southern Great Plan (SGP) regon. A detaled descrpton of these three management practces can be found n Teague et al. [15]. The lfe cycle we consder ncludes the entre producton perod from the start of the breedng season n Aprl, to the pont when the weaned calves are sold, n November of the followng year as depcted n Fgure 1. Cattle transportaton from the ste of cow-calf producton to the next phase of producton s not consdered. The functonal unt, to whch all the envronmental loads n the LCA are related, s generally defned as 1 kg lve or carcass weght f the entre beef producton systems are studed, whch ncludes cow-calf, backgroundng and fnshng systems [11]. As the boundary of our LCA study s lmted to the calf-cow producton system, we defne the functonal unt as one marketed beef calf as n Ogno et al. [16], so that our result can be easly compared wth lterature value from other regons. In addton, to capture the stockng densty dfferences of dfferent grazng strateges, we have also used one hectare of rangeland as the alternatve functonal unt. Beef producers and regonal extenson experts have descrbed the current cow calf producton condtons n SGP area as follows. Most ranchers n the SGP area follow a breedng season from Aprl to August, resultng n calf brths from January to May. Weanng occurs from September to November. The producton cycle of the year-1 cohort overlaps wth the producton cycle of the year-2 cohort. Specfcally, durng the breedng perod and part of the gestaton perod, the cows stll feed the prevous cohort of calves. Smlarly, durng the lactaton perod of the current cohort, the cows wll breed and become pregnant wth the next cohort. Based on the contemporary producton data, Fgure 1a c descrbe the tmelne of the calf producton cycle for the mature cows, 1st-year hefers and 2nd-year hefers respectvely. Note that n Fgure 1b, the female cattle start as 1st-year hefers, but turn nto 2nd-year hefers at ther second breedng season and eventually become mature cows at the end of our defned producton cycle. Smlarly, n Fgure 1c, the female cattle start as 2nd-year hefers, then become mature cows after weanng ther frstborn calves. For all female cattle for reproducton purpose, for smplcty

4 Sustanablty 2015, we assume the same average pregnancy rate, lactaton rate and weanng rate as 90%, 86% and 82%, respectvely. (a) (b) Fgure 1. Cont.

5 Sustanablty 2015, (c) Fgure 1. (a) Tmelne of a producton perod for female cattle startng as mature cows; (b) Tmelne of a producton perod for female cattle startng as 1st-year hefers; (c) Tmelne of a producton perod for female cattle startng as 2nd-year hefers. For demonstraton purpose a unform gestaton perod for all cows from July to the followng Aprl s assumed. Take Fgure 1a, the producton tmelne for mature cows for example, based on a 283-day pregnancy ths gves 133 days of overlap between pregnancy and lactaton and 150 days of pregnancy wthout lactaton. Pror to each gestaton perod, we assume a 98-day perod of lactatng only for the prevous cohort. On average, pregnancy rate, lactaton rate and weanng rate are 90%, 86% and 82%, respectvely. Thus, n ths producton cycle descrbed n Fgure 1a, 82% of the cows spend 196 days lactatng but not pregnant, 150 days pregnant but not lactatng, and 266 days both lactatng and pregnant, totalng 612 days. Among the rest, 14% of cows ddn t calve, based on the 86% lactatng rate. It was assumed that all cows that ddn t calve were nether lactatng nor pregnant durng the entre 612-day perod. About 4% of cows gve brth but do not rase a calf to the weanng stage. We assume these cows dd not lactate durng the entre 612-day perod. Based on Fgure 1 we assume these 4% of cows are pregnant for a perod of 416 days n the producton cycle and are nether lactatng nor pregnant for the remanng 196 days. Of the 332 weaned calves, except for the 53 that are retaned as replacement hefers, the rest of the calves are sold mmedately after weanng n November. The replacement hefers get bred for the frst tme durng the next year s breedng season, and wll be pregnancy checked at around 19 months old. They typcally reach ther mature weght when they wean ther frstborn calves. After that they wll stop growng and wll reman on the ranch untl ten to eleven years old. Fgure 1b, c resemble Fgure 1a n the breedng season, gestaton perod and lactaton perod. However, as 1st-year hefers are growng through the producton season, and 2nd-year hefers also grow part of the producton season, actvtes

6 Sustanablty 2015, that attrbutes to enterc methane emsson are dfferent for 1st, 2nd-year hefers and mature cows even at the same producton stage, as marked n Fgure 1a c. In SGP regon, the cattle are grazed on natve prare pasture 100% of the tme. Supplemental hay s rarely used n the SGP regon, except for the years of severe drought, therefore we wll not take t nto account n our study. In ths regon cotton seed meal s commonly used as a proten supplement durng the wnter when grass proten s low and we calculate the GHG emssons assocated wth ths source of feed. The cows, 1st and 2nd-year hefers and bulls are fed klograms (2 pounds) of supplemental proten per head per day for 120 days. Descrptons of the breakdown of cattle numbers for LC, HC and MP grazng can be found n Table 1.The total area of the representatve farm n SGP area s defned as 4000 hectare. Gven that 1 Anmal Unt (AU) equals 450 kg, the stockng rates for LC grazng herd n SGP area s 14 AUs 100 ha 1, whle those for HC and MP are both 27 AUs 100 ha 1, as descrbed n Teague et al. [15]. In SGP regon, the Typcal Anmal Mass (TAM) for cows and bulls are 500 and 900 kg head 1 respectvely. The 1st-year hefers weght 408 kg on average durng the producton cycle, as they start to breed n Aprl at 12 months of age weghtng 315 kg, and reach the mature cow weght of 500 kg by the tme they wean ther frstborn calves (Fgure 1b). The 2nd-year hefers start at 431 kg at begnnng of the 2nd breedng season and ther weghts stablze at 500 kg after weanng ther frstborn calves (Fgure 1c). Therefore the weghted average weght for 2nd-year hefer s 487 kg durng the producton cycle. The TAM for calves s 40 kg at brth and 220 kg at weanng age, so we used the average value of 130 kg per head. Based on expert s opnon, the herd under LC grazng s comprsed of 299 mature cows, 53 1st-year hefers, 53 2nd-year hefers, and 13 bulls. Based on the 82% weanng rate, 332 calves are weaned under LC grazng strategy. The proporton of cows, hefers, bulls and calves under HC and MP grazng are the same as the LC system. Table 1. A descrpton of the representatve farms under three dfferent grazng strateges. Parameters Lght Contnuous (LC) Heavy Contnuous (HC) Mult-Paddock (MP) Total number of cattle cows st year hefers nd year hefers calves bulls Typcal Anmal Mass (TAM) cows 500 kg [1.11 AU] 500 kg 500 kg 1st year hefers 408 kg [0.91 AU] 408 kg 408 kg 2nd year hefers 487 kg [1.08 AU] 487 kg 487 kg calves 130 kg [0.29 AU] 130 kg 130 kg bulls 900 kg [2.00 AU] 900 kg 900 kg Total AUs Total hectare 4000 ha 4000 ha 4000 ha Stockng rate (AU 100 ha 1 ) Pregnancy rate 90% 90% 90%

7 Sustanablty 2015, Parameters Lght Contnuous (LC) Table 1. Cont. Heavy Contnuous (HC) Mult-Paddock (MP) Calvng rate 86% 86% 86% Weanng rate 82% 82% 82% Breedng Season Aprl to August Aprl to August Aprl to August Brth Next January to May Next January to May Next January to May Weanng Next September to November Next September to November Next September to November Weanng age 7 months 7 months 7 months Supplemental hay Rarely used, except for Rarely used, except for Rarely used, except for severe drought years severe drought years severe drought years Supplemental proten (cotton seed meal) 1 2 pounds cow for 120 days 1 day 1 2 pounds cow for 120 days 1 day 1 2 pounds cow for 120 days 1 day Energy use desel 6.07 gallons/ha 6.07 gallons/ha 6.07 gallons/ha gasolne 0.74 gallons/ha 0.74 gallons/ha 0.74 gallons/ha LP gas 1.62 gallons/ha 1.62 gallons/ha 1.62 gallons/ha electrc kwh/ha kwh/ha kwh/ha Fertlzer use N P Pregnancy rates as well as the lactatng and weanng rates across three dfferent grazng strateges are assumed the same as the average correspondng rates descrbed prevously n ths secton. Ths assumpton s based on prevous lterature fndngs, whch reported no dfferences n pregnancy rate for cows assocated wth dfferent stockng-rate treatments [17 20], cows mantaned on contnuous grazng and varable rotatonal grazng treatments [21,22], and cows under contnuous and rotatonal stockng at dentcal stockng rates [23,24] GHG Emssons In ths secton we descrbe the methodologes used n GHG emsson calculatons. The envronmental loads assocated wth beef cow-calf producton are anmal body, supplemental feed producton and anmal management. Specfcally, fve components of GHG emsson on a typcal SGP cow-calf farm were ncluded: enterc methane emsson, manure methane emsson, manure ntrous oxde (N2O) emsson, supplemental proten CO2 emsson and GHG emsson from farm energy use and fertlzer use. For the frst three components, GHG emssons were frst calculated usng IPCC [25] on a per producton cycle bass, then converted to the annual bass to be compatble n tme frame wth GHG emssons the last two components and carbon sequestraton. In addton, all gases were converted to CO2 equvalents (CO2e) to account for the global warmng potental, where CO2 = 1, CH4 = 25 and N2O = 298 [26]. To compare GHG emsson wth carbon sequestraton, CO2e s also converted to carbon equvalents (CE), where CO2e = 12 / 44 CE, accordng to Envronmental Protecton Agency [EPA] [27].

8 Sustanablty 2015, Enterc CH4 Based on IPCC [25], CH 4enterc = EF N (Unt: kg CH4 per producton cycle), where N s the number of anmals n subcategory and EF s the emsson factor for the anmals n subcategory wth the unt of measurement beng kg CH4 head 1 year 1. Here = 1denotes cows that are both lactatng and pregnant; = 2 denotes cows that are lactatng but not pregnant; = 3denotes cows that pregnant but not lactatng; = 4 denotes cows that are nether pregnant nor lactatng; = 5 denotes 1st-year hefers that are growng only; = 6 denotes 1st-year hefers that are growng and pregnant; = 7 denotes 2nd-year hefers that are growng and lactatng; = 8 denotes 2nd-year hefers that are growng, pregnant and lactatng; = 9 denotes bulls and = 10 denotes calves. Accordng to IPCC [25], the Ter 2 method should be used f enterc fermentaton s a key source category for the anmal category that represents a large porton of the country s total emssons. Clearly, Ter 2 approach should be adopted for beef cattle. Usng Ter 2 approach EF s calculated as: EF = GE Ym Day / (1) where Ym s the methane converson factor, whch s the percent of gross energy n feed converted to methane. Based on IPCC [25], Y m = 6.5 ± 1.0. Lower bound s more approprate for feed wth hgh dgestblty and hgh energy value, and vce versa. Wthout better nformaton, the mean value s chosen for our baselne analyss. For calves fed entrely on mlk, IPCC [25] specfed that Y m = 0. However, as ther rumens develop, the calves also starts to emt methane, IPCC [25] dd not provde any nformaton on Y m for ths category. Unaware of any enterc methane emsson data for calves n SGP regon, we wll use the methane emsson rate measured by Westberg et al. [28] for four calves on pasture owned by Washngton State Unversty Department of Anmal Scences. At the tme of measurement, these four sucklng calves were at 4 months n age weghng 206 kg on average, and were temporarly separated from ther mothers. Snce the average calf weght s 130 kg n our producton cycle, we wll proportonally adjust the measurement of Westberg et al. (2001), whch was 2.2 g day 1 calf 1, by 0.75 TAM [25]. The average daly enterc emsson for calves s thus calculated as / = 1.6 g day 1 calf 1 n our case. Factor (MJ/kg CH4) stands for the energy content of methane. Day denotes the number of days that the anmals actually stayed on the farm; GE s the gross energy ntake (Unt: MJ head 1 day 1 ), whch s further defned as: GE ( m a l p )/ g = NE + NE + NE + NE REM + NE / REG / DE (2) m here NE s the net energy for mantenance, requred by the anmal so that body energy s nether ganed m 0.75 or lost, whch s calculated by NE = Cf TAM. Here, coeffcent Cf takes the value of for lactatng female cattle ( = 1, 2, 7,8 ), for non-lactatng females ( = 3, 4,5, 6 ) and for bulls ( = 9 ). a m Next, NE = Ca NE stands for the net energy for actvty, or the energy needed by the anmal to obtan food, water and shelter. It s based on the feedng stuaton rather than the feed tself. For cattle confned n a small area such as a barn, C a = 0; for cattle confned n areas wth suffcent forage such

9 Sustanablty 2015, as a pasture C a = 0.17 ; for cattle grazng n an open rangeland, C a = In our study the cattle graze an open rangeland, thus, C a = 0.36 s chosen. l For lactatng cows ( = 1, 2, 7,8 ), NE = Mlk ( fat) s the net energy for lactaton, where Mlk s the amount of mlk produced (kg day 1 ) and fat s the fat content of mlk (%). Accordng to EPA [29], the monthly lactaton estmates for beef cows from January to December are respectvely, 1.5, 2.3, 3.9, 5.4, 6.2, 6.0, 5.3, 4.2, 3.1, 2.0, 1.4 and 1.3 kg beef cow 1 day 1. Accordng to Fgure 1, f the calves are born n early Aprl and weaned late November, the average daly mlk producton would be 4.21 kg day 1. Also accordng to EPA [29], there s 4 per cent of fat n mlk. For pregnant cows p p m ( = 1,3,6,8), NE s the net energy requred for pregnancy, where NE = 0.1 NE. g For hefers, NE ( = 5,6,7,8 ) s the net energy for growth, defned as NE = [ BW / ( C MW )] WG, where BW s the TAM of the hefer (kg); C s a coeffcent g wth a value of 0.8 for hefers; MW s the mature lve body weght of an adult female n moderate body condton (kg), or TAM of the cow; WG s the average daly weght gan of the hefers, as demonstrated n Fgures 1b and 1c for hefers at dfferent lfe stages. Feed dgestblty ( DE ) for cattle ranges from 45 % to 55% for crop byproducts and range lands and 55% to 75% for good pastures, good preserved forages and gan supplemented forage based dets. In Southern Great Plans, we use the ntermedate dgestblty value for rangeland, that s DE = 50. Fnally, we calculate REM and REG based on DE%. REM s the rato of net energy avalable n det for mantenance to dgestble energy consumed: REM DE DE 25.4 / DE = + (3) REG s the rato of net energy avalable n det for growth to dgestble energy consumed: / REG = DE + DE DE (4) Manure CH4 On a cow-calf farm, cattle drectly depost dung and urne on the natve prare pasture where they graze all year long. Storage and treatment of manure, whch occur very often when large anmals are managed n a confned area, such as the feedlot, s not applcable to the cow-calf producton n SGP regon. Based on IPCC [25], compared to manure stored or treated as a lqud, manure deposted on pastures and rangelands tend to produce less manure CH4. To calculate manure methane emsson, the method provded by ICF Consultng (ICF) [30] s used, whch s very smlar to the Ter 2 method n IPCC [25], but s more nformatve n that nstead of treatng the North Amercan regon as a whole, t treats each state n U.S. separately. Overall, the manure methane emsson can be calculated as: CH4manure = VS B0 MCFj WSj (5) j where VS stands for the volatle sold produced by all the anmals n subcategory per year and t can be computed from: VS = N TAM VScoeff (6)

10 Sustanablty 2015, Accordng to ICF [30], VS coeffcent ( VS coeff ) s 2.6 (unt: kg VS/kg anmal mass/year). Thus, TAM VScoeff values for calves, hefers, cows and bulls can be calculated as 338, 819, 1300 and 2340 (kg/anmal/year). Note that EPA [29] also provdes TAM VScoeff as a sngle value VS (kg/anmal/year) for the year As per EPA [29], for two avalable categores, hefers and cows, VS took the value of 1013 and 1589 respectvely n Oklahoma, and 1053 and 1664 respectvely n Texas. These estmates are about 25% hgher than the estmates usng the ICF [30] method. Therefore our estmates are adjusted by a factor of 1.25 to obtan the updated estmates for all four categores. The adjusted TAM VScoeff values for calves, 1st-year hefers, 2nd-year hefers, cows and bulls are 423, 1326, 1583, 1625 and 2925 (kg/anmal/year) respectvely. B 0 s the estmate of maxmum methane producng capacty of U.S. lvestock. For beef NOT n feedlots, B0 = (Unt: kg per kg VS). Note that to mantan the consstency of unt we have changed the value of B0 accordngly, whch, accordng to ICF [30], took the value of 2.72 (Unt: ft 3 per lb VS). Also note that 1 ft 3 = lbs [30]. WS s the percent of anmal s manure managed n manure system j. For all anmal categores we assume a 100% pasture/range/paddock manure system, for whch the methane converson factor (MCF) for manure system s 1.4% n both Texas and Oklahoma Manure N2O Accordng to IPCC [25], the N2O emssons generated by manure n the system pasture, range and paddock occur drectly and ndrectly from the sol. Besdes urne and dung, no other forms of manure such as organc N addton and synthetc fertlzer are appled on the natve pasture. Therefore, drect N2O emssons from urne and dung nputs to grazed sols can be calculated as (Unt: kg N2O/producton perod): j NO 2 drect = Fdrect EF3 44 / 28 (7) where F drect (Unt: kg N/producton perod ) s the amount of non-volatlzed ntrogen excreted by grazng anmals on pasture, range and paddock n a producton perod. The value of F drect can be estmated by ICF [31] method, that s: F = 0.8 N ( TAM /1000) K Day drect N Kjeldahl Ntrogen per day per 1000 kg mass, or K N (Unt: kg N/day), takes the value of Fgure 1 shows that calves stay on the pasture for a total of 462 days (231 days for each cohort) durng the producton perod, whle the other subcategores of anmals are assumed to stay on the farm throughout the producton perod. EF 3 (Unt: kg N2O/kg N) s the emsson factor for N2O emssons from urne and dung deposted by grazng anmals on pasture, range and paddock. Accordng to IPCC [25], EF 3 takes a default value of 0.02 wth uncertanty range between and Wthout better nformaton, we wll choose the default value of EF 3 for all three grazng strateges. In addton to drect emssons of N2O, emssons of N2O also take place n two ndrect channels, (1) through the volatlzaton of ntrogen as NH3 and oxdes of ntrogen and deposton of these gases and ther products back onto sols, and (2) through leachng and runoff of ntrogen. Based on IPCC [25], (8)

11 Sustanablty 2015, leachng and runoff are unlkely to occur for dryland regons, where precptaton s lower than evapotranspraton most tme of the year. Therefore we assume that under the sem-ard clmate of SGP regon, leachng and runoff do not occur on cow-calf farms, as they are typcally non-rrgated and unfertlzed. Thus, ndrect N2O emssons can be calculated as: NO 2 ndrect = Fndrect EF4 44 / 28 (9) where F ndrect s the amount of ntrogen deposted by grazng anmals on pasture, range and paddock (Unt: kg N/ producton perod), whch can be calculated as: F = φ N ( TAM /1000) K Day ndrect N where φ s the fracton of volatlzed ntrogen as NH3 and NOx, whch takes a default value of 0.2 wth uncertanty range between 0.05 and 0.5 [27]. EF 4 s the emsson factor for N2O emssons atmospherc deposton of ntrogen on sols, whch takes a default value of 0.01 wth uncertanty range between and 0.05 [27]. Wthout further nformaton, we wll assume default values for both φ and EF Proten Supplement In the SGP regon cottonseed meal s used as the man source of supplemental proten. The majorty of cotton seed avalable as supplement for beef cattle n Texas and Oklahoma s from central pvot rrgaton (pers. comm. wth Dr. Paul DeLaune, Envronmental Sol Scentst, Texas A&M AgrLfe Research, Vernon, Texas.). Accordng to van Zest et al. [32], the yeld of cotton s dvded nto fbers (38%) and seed (62%), of whch 5% of seed s reserved for replantng. The yelds of ths cotton seed from central pvot rrgaton for ndustral producton purpose s 3030 kg ha 1 and feld producton GHG emssons s 884 CE ha 1 [33]. We assume that 57% of the GHG emsson s attrbutable to the cottonseed for non-reproducton purpose and calculated the emssons at kg CE ha 1. Thus, the feld level GHG emsson for cottonseed producton s estmated as 0.17 kg CE per kg of cottonseed n the feld. Energy requred for crushng cottonseed s 1.25 mm BTU per ton of cottonseed crushed, when natural gas s used as the man source of thermal energy [34]. Accordng to EPA [27], the carbon content coeffcent s kg C per mm Btu for natural gas; therefore, the GHG emsson s kg CE per ton of cottonseed crushed, or 0.02 kg CE per kg of cottonseed crushed. Together, GHG emsson s 0.19 kg CE per kg of cottonseed. For 1000 kg of cottonseed yeld, ndustry-wde yelds are 160 kg cottonseed ol, 455 kg cottonseed meal, 270 kg husks, 83.5 kg lnters and 31.5 kg beng lost. If we assume that 45.5% of GHG emsson on cottonseed s attrbutable to cotton seed meal, then GHG emsson s 0.19 kg CE per kg of cottonseed meal produced. Accordng to U.S. Department of Agrculture (USDA)-Natonal Agrcultural Statstcs Servce (NASS) Crop Producton report released n August 12, 2015, the 2014 harvested area for upland cotton producton n Texas was 1,861,519 hectare (4,600,000 acres). In addton, based on the statstcs provded by Natonal Cattlemen s Beef Assocaton [35], for the year 2014 there were 4350,000 cows that calved n Texas. Gven that each cow s fed kg of supplemental proten per head per day n wnter for 120 days, and that the yelds of ths cotton seed from central pvot rrgaton as 3030 kg ha 1 as we assumed above, together the calved cows consume 8.4% of the annual cotton harvested f they were planted under central pvot rrgaton. Therefore the land currently used for cotton (10)

12 Sustanablty 2015, producton s suffcent to meet the cow-calf farm s proten supplementaton requrement. As no land use converson to cotton producton s necessary, GHG emsson assocated wth land use change effect wll not be consdered n ths paper Energy Use CO2 Ryan and Tffany [36] reported fuel related energy expenses of $10.24 per head for cow-calf operators n 1995, wth the energy use breakdown data as 6.07, 0.74, 1.62 gallons for desel, gasolne and LP gas respectvely, and kwh for electrc. Based on 124,884 Btu per gallon and 3413 Btu per kwh, the energy use were 758,046, 92,414, 202,312 and 202,186 Btu for desel, gasolne, LP gas and electrc respectvely. Accordng to the converson unt provded n Del Grosso, Walsh and Duffeld [37], the GHG emsson from desel, gasolne, LP gas and electrc are 54.89, 6.48, 12.62, kg CO2 equvalents respectvely on a per cow bass GHG Emssons Senstvty Analyss GHG emsson calculaton methods for the baselne scenaro were provded n the prevous sectons. For contnuous grazng strateges (LC and HC) only the baselne scenaros were consdered. To gan a better nsght nto rotatonal grazng, both the baselne scenaro and several alternatve scenaros were consdered. Compared to contnuous grazng strateges, rotatonal grazng mproved grass composton [15] and forage qualty [38]. Therefore two factors related to forage qualty are allowed to alter: the methane converson factor Y m n Equaton (1) and the feed dgestblty factor ( DE ) n Equatons (3) and (4). In addton, under rotatonal grazng strategy the anmals are confned for a short perod to a grazng paddock that s much smaller compared to that n contnuous grazng. Thus, the lower a value of Ca s also chosen, whch s the coeffcent to calculate NE, the energy needed to obtan food, water and shelter. Overall, for rotatonal grazng we consdered four scenaros: () the baselne scenaro; () lower methane converson factor and hgher feed dgestblty ( Y m = 5.5 and DE = 60 ), lower energy n obtanng food by choosng the average value of Ca between pasture grazng and open rangeland grazng ( C a = ); () lower methane converson factor and hgher feed dgestblty ( Y m = 5.5 and DE = 60 ), energy to obtan food the same as baselne ( C a = 0.36 ); and (v) methane converson factor and feed dgestblty the same as baselne ( Y m = 6.5 and DE = 50 ), and lower energy to obtan food ( C a = 0.265) Carbon Sequestraton of Rangeland Sol Organc Carbon (SOC) Stock Wth the sol organc matter (%) avalable n Teague et al. [15], we calculated the sol organc matter as: SOM = denstyk volume SOM k% (11) k Here k = 1 refers to sol of depth 0 15 cm; k = 2 refers to sol of depth cm and k = 3 refers to sol of depth cm. The values of densty 1 for LC, HC and MP were obtaned from Teague et al. [15]

13 Sustanablty 2015, as 0.98, 1.06 and 0.91 Mg m 3. Bulk densty at other sol depth levels was not measured n Teague et al. [15]. Therefore the values of densty2 / densty 1 and densty3/ densty 1 were frst obtaned from the average sol densty values of the 6-year, 26-year and 60-year old restored grassland as measured by Potter, Torbert, Johnson, and Tschler [39]. Assumng the bulk densty n sol depth of 0 15 cm (average bulk densty among bulk densty of sol depth of 0 5 cm, 5 10 cm and cm) as 100%, then the bulk densty n the sol depth of cm (weghted average bulk densty of cm and cm) and cm (weghted average of cm and cm) are 115.1% and 120.8% respectvely, takng the average of the three grassland stes. Percentages of sol organc matter (SOM) at varous sol depths are denoted as SOM k %, whch can be obtaned from Teague et al. [15] for LC, HC and MP grazng strateges. The correspondng SOC can be computed gven that SOM contans 58% carbon [40] Carbon Sequestraton Teague et al. [15] studed three ranches practcng LC, HC and MP strateges n each of three adjacent countes. The same grazng strategy has been practced for at least 9 years before the measurements were taken. Of the three ranches currently practcng MP grazng, two were converted from HC to MP, and one was orgnally under LC. The converson occurred 10 years prevously for the frst two ranches and 20 years prevously for the thrd ranch before the year of measurement. One of the lmtatons of ths study s that the carbon stock measurement from prevous years s unavalable. The farms wth LC, HC and MP practces for each county, however, are located rght across the fences from one another or nearby. Therefore, t s reasonable to assume that the MP practcng farms ntally had the same SOC stock as the neghborng HC or LC farms f the converson had not occurred. Smlar to Stephenson et al. [41], we frst establsh the carbon stock for LC and HC as the benchmark, and consder the relatve changes n carbon stock when the converson to dfferent grazng practces occurred. We consdered three transtons: HC to MP, HC to LC and LC to MP. For senstvty analyss purposes, three tmng scenaros are also consdered, namely transton occurrng 10, 15 and 20 years pror to transton. To calculate carbon sequestraton for each scenaro, we adopted the methods used n Gascogne et al. [42] and dvded the dfference n carbon stock by the number of years passed snce converson occurred. For example, f durng the past T ( T = 10,15, 20 ) years the total carbon stock of up to 60 cm sol depth has ncreased from CS 1 to CS 2 Mg ha 1 due to the change n management practce, then ths mples an annual sequestraton rate of ( CS2 CS1)/ T Mg ha 1 per year. It s worth notng that when applyng ths method to the data n Potter et al. [39], t generated 412 kg ha 1 sequestraton rate per year, very close to ther regresson result of 447 kg ha 1 sequestraton rate. When there s no management mprovement, for example, f HC or LC s always n practce wthout converson to MP, then we assume the sol reaches an equlbrum and the C sequestraton s zero [43].

14 Sustanablty 2015, Results and Dscusson 3.1. GHG Emssons Table 2 demonstrated GHG emssons for LC, HC and MP grazng strateges. Note that for the MP baselne scenaro, we assumed the same parameters as contnuous grazng n LCA approach. Such assumpton s later on releved n MP alternatve scenaros to account for the potental mprovement n grass qualty and the less requred energy as anmals are confned n a much smaller paddock under MP scenaro. On a per calf bass, the total emsson s kg CO2e calf 1 year 1 for LC, HC and MP baselne scenaro, due to the same pregnancy and weanng rates assumed for the three grazng strateges based on prevous lterature observatons. Though no dfference n pregnancy rates were found under dfferent stockng rate and management strategy, Arthngton, Bohlen and Roka [18] showed pounds of calf weaned per acre of dedcated land was greater for hgh compared to medum and low stockng rates. Therefore the three grazng systems wll dffer n carbon emssons per pounds of calf weaned. Due to lack of feld measured data n SGP area and no comparable lterature values, pounds of calf weaned wll not be used as the functonal unt n our paper. To make a comparson wth the carbon sequestraton, total carbon emsson s also computed on a per hectare bass, for whch the value for LC s proportonally lower than those for HC and MP, due to the dfferent stockng rates. Table 2. Greenhouse gas (GHG) emssons for farms of dfferent management systems. GHG Emsson (kg CO 2 e calf 1 year 1 ) LC HC MP Baselne MP Alternatve 1 MP Alternatve 2 MP Alternatve 3 Enterc CH 4 emsson Manure CH 4 emsson Manure N 2 O emsson Proten Supplement Energy Use CO Total Emsson Total Emsson (kg CE calf 1 year 1 ) Total Emsson (kg CE ha 1 year 1 ) For a cow-calf farm n Upper Mdwestern U.S. that provdes 75 calves for beef producton, Pelleter et al. [44] reported a total GHG emsson of 599 ton CO2e, whch s equvalent to kg CO2e calf 1. Ths overall emsson s only 0.6% lower than the value calculated n ths paper. However, on a cow-calf system n Japan, Ogno et al. [16] reported a total GHG emsson of 4550 kg CO2e calf 1 whch s 43.3% lower than the GHG emssons found n ths study. Among the sources of GHG emsson presented n Table 2, we can calculate the emsson dstrbuton as 79.6% for enterc CH4, 1.9% for manure CH4, 15.9% for manure N2O, 1.2% for proten supplement and 1.4% for energy use. Clearly enterc methane accounts for the majorty of all emssons, wth emsson from manure (CH4 and N2O) rankng next. GHG emssons from the two other sources only

15 Sustanablty 2015, account for less than 3% of the total emsson, whch s almost neglgble. Enterc methane accounts for 43.4% of the total GHG producton for the cow-calf farm n Upper Mdwestern U.S. [44] and 61.2% of the total emsson n Japan [16], both of whch are much lower than our estmaton. The vast gaps n percentage of GHG emssons from dfferent subcategores of cow-calf producton can be explaned largely by the dfferent producton practces resultng from regonal varaton n clmate and grazng condtons [4]. For example, the representatve farm studed n Pelleter et al. [44] lkely used supplemental forage to sustan cows durng wnter. However, n the SGP regon we studed, cows graze year round on natve pasture and generally no harvested forage s necessary for the wnter months. As a result, feed producton, though not needed for SGP regon, s the second largest emsson category n Pelleter et al. [44], accountng for 32.9% of the total emsson. Smlar dfferences can be found n the cow-calf system n Japan [16], whch reported 18.4% and 8.3% of the total emsson attrbutable to feed producton and feed transport. Feed transport, whch s not applcable n our study, s necessary n the study of Ogno et al. [16], due to the 25% of mported feed from U.S. and Chna. The major reason that SGP regon has much hgher GHG emssons for cow-calf producton s due to the much hgher enterc CH4 emssons, whch s caused by relatvely lower feed qualty on the unfertlzed rangeland compared to that on the fertlzed pasture. As our senstvty analyses on MP grazng show, f we take account of the potental of grass qualty mprovement by MP grazng [38], as well as the reduced grazng energy due to much smaller paddock szes of MP, then MP grazng reduces the total GHG emsson by 33% to kg CO2e calf 1 year 1, as reflected by the MP Alternatve 1 scenaro n Table 2. Even f the reduced grazng energy potentally caused by MP grazng s not accounted for, the total GHG wll stll be lowered to kg CO2e calf 1 year 1 (MP Alternatve 2). However, f the beneft for MP grazng n LCA analyss s only lmted to reduced grazng energy whle the grass qualty remans the same, then GHG emsson wll be lowered by less than 5% from the baselne scenaro (MP Alternatve 3). These results underlne the mportance of grass qualty mprovement n reducng methane CH4 emsson, and n return the total GHG emsson. Wth the potental to ncrease grass qualty and dgestblty [38], MP grazng could lead to a bg reducton n GHG emsson. Note that our results on methane reducton by MP grazng s consstent wth the feld study conducted by DeRamus et al. [45], who measured methane emssons of cattle on dfferent grazng management practces usng methane collecton equpment, and found a 22% reducton n annual methane emsson from MP grazng when compared wth contnuous grazng Carbon Sequestraton Table 3 shows that the SOC stock up to 60 cm depth for LC, HC and MP management practces are 122.6, 93.9 and Mg ha 1 respectvely. Therefore, MP grazng led to the hghest SOC stock, wth LC slghtly laggng behnd by only 5%. HC has the lowest SOC stock, whch s 27% lower than that for MP. Note that our results on SOC stock were comparable to the fndngs of Potter et al. [39], also n SGP regon, where the SOC stock for the 6-year, 26-year and 60-year restored grassland, whch were orgnal agrcultural land, were 110.0, and Mg ha 1 respectvely. We can see that our SOC stock value for the MP scenaro s very close to that of the 60-year restored grassland; even SOC stock for LC s much hgher than the 6-year and 26-year restored grassland. However, SOC stock for HC

16 Sustanablty 2015, scenaro fell far behnd, whch suggests HC grazng generated very lttle or no carbon sequestraton over the tme. Ths observaton concdes wth that of Follett and Reed [46], whch showed mproved grazng management, wth ts ntroducton of legumes, control of undesrable speces and enhanced grass productvty, generally ncreases carbon sequestraton. Meanwhle, moderate contnuous grazng leads to a hgher margn of C sequestered over GHG emsson than heavy contnuous grazng [12]. Table 3. Carbon stock for farms of dfferent management systems. GHG Sequestraton LC HC MP Sol bulk densty (Mg m 3 ) 0 15 cm 0.98 a 1.06 a 0.91 a cm cm SOM (%) 0 15 cm 5.24 a 3.76 a 5.72 a cm 3.55 a 2.45 a 4.00 a 30 60cm 2.09 a 1.49 a 2.48 a SOM (Mg ha 1 ) 0 15 cm cm cm SOM (0 to 60 cm, Mg ha 1 ) SOC (0 to 60 cm, Mg ha 1 ) a Values from Teague et al. [15]. From Table 4 we can see that carbon sequestraton rates vared greatly for dfferent scenaros, wth hghest C sequestraton rate for the 10-year transton scenaro and the lowest C sequestraton for the 20-year scenaro. Ths s because f the same accrual of SOC stock occurs under a shorter perod of tme, then SOC accumulaton rate for each year s n return hgher, whch means hgher SOC sequestraton. Note that SOC stock may not ncrease wth the same accrual rate each year. As ponted out by Follett and Reed [46], relatvely hgh C sequestraton occurs on recently restored rangelands (up to 2.75 Mg C ha 1 year 1 ) whle lower C sequestraton occurs (0 to 1.6 Mg C ha 1 year 1 ) on rangelands managed the same way over the long term. Gven any transton perod, the hghest sequestraton always occurred durng the transton from HC to MP, whle the sequestraton rate under transton from HC to LC ranks second, generatng slghtly lower C sequestraton, and the sequestraton rate from LC to MP s much lower, generatng only 20% of the hghest C sequestraton rate. Therefore C sequestraton rate s closely ted to the ntal land use practce. Transtonng from LC to MP generates a C sequestraton rate of 330 kg C ha 1 year 1, whch s consstent wth most carbon sequestraton rates reported by the lterature. For example, Stephenson et al. [41] reports a C sequestraton rate of MP grazng les wthn the range of 120 to 400 kg C ha 1 year 1, and n the NGP regon, Lebg et al. [12] report C sequestraton rates of 390 to 460 kg C ha 1 year 1 for the unfertlzed natve prare and fertlzed crested wheatgrass. Soussana et al. [47] also obtaned annual C sequestraton rates between 191 and 491 C ha 1 year 1 when modelng the European grassland management systems. After convertng from HC to MP, the C

17 Sustanablty 2015, sequestraton rate was estmated as a much hgher value of 1765 kg C ha 1 year 1, whch s consstent wth the fndng of Soussana et al. [48], who reported a C sequestraton rate of 2400 ± 700 kg C ha 1 year 1. Ths s because when poor management lowered SOC stock over the tme, a transton to an mproved practce such as MP wll ncrease SOC stock at a hgher rate [43]. Table 4. Possble (net) carbon emsson and sequestraton for dfferent transton scenaros. C Emsson/Sequestraton (kg CE ha 1 year 1 ) 10-Year 15-Year 20-Year C Sequestraton HC MP HC LC LC MP C Emsson HC MP HC LC LC MP Net C Emsson HC MP HC LC LC MP To be conservatve on the GHG mtgaton potental of MP grazng, GHG emsson from only MP baselne scenaro n Table 2 s used to estmate carbon balance. Smlar to Lebg et al. [12], who observed that unfertlzed natve prare was a net snk for GHGs, our analyss also ndcated that cow-calf farms convertng from contnuous to rotatonal grazng n SGP regon could be ether net carbon snks or low carbon sources for decades. For example, under the ntermedate 15-year scenaro, cow-calf farms convertng from HC to MP, from HC to LC and from LC to MP are lkely net snks for GHG wth a net C sequestraton rate of , and 89.5 kg C ha 1 year 1 respectvely. Even the most conservatve 20-year scenaro (Table 4), cow-calf farms convertng from HC to MP and from HC to LC are net snks for GHG wth a net C sequestraton rate of and kg C ha 1 year 1. A transton from LC to MP wll generate a low net carbon emsson rate of 20.5 kg C ha 1 year 1 for the 20-year scenaro. Therefore, t s worth notng that even though the GHG emssons n SGP regon are hgher on a per calf bass compared to the values reported n other regons of the world [16,44], net GHG emssons are lkely negatve when we take the carbon sequestraton nto account (Table 4). Ths s consstent wth results from NGP regon reported by Lebg et al. [12] where, when usng a modest annual SOC sequestraton rate of 0.17 tons C ha 1 wth the contnuously grazed forage base, both heavy and moderately stocked grazng strateges produced substantal carbon snks of and tons CO2e ha 1 year 1 respectvely. Overall these systems yelded and tons CO2e kg 1 anmal gan whle the enterc methane was reported to be and tons CO2e ha 1 year 1. If we assume a sol C sequestraton rate of 330 kg C ha 1 year 1 as from LC to MP as reported n the 20-year scenaro (Table 4), t wll take an addtonal 116 years for the current SOC stock under MP practce to reach that of the natve prare reported by Potter et al. [39], whch averaged Mg C ha 1 n

18 Sustanablty 2015, central Texas. Lkewse f the sequestraton rate of 440 kg C ha 1 year 1 from LC to MP as reported n the 15-year scenaro s assumed, then t wll take 87 years to reach the SOC level of the natve prare. Thus, the upward trend n C sequestraton wll lkely contnue for a number of decades. When assumng the C sequestraton rate of 1765 kg C ha 1 year 1 under 20-year scenaro, whch occurred durng the transton of HC to MP, t only takes an addtonal 38 years for the SOC stock under HC grazng to reach the SOC stock value of an average natve prare, or 23 years under the 15-year scenaro. Ths concdes wth the concluson of Smth [43] and suggests that ths hgh carbon sequestraton rate s unlkely to last over the long term. However, for decades after convertng from the contnuous grazng to MP grazng practce, the contrbuton of GHG emsson by cow-calf farm n the beef producton lnk s non-sgnfcant, non-exstent, or negatve relatve to carbon sequestraton rates for rumnants feedng solely on grazed perennal pastures. Ths s contrary to many commonly reported LCA analyses ndcatng that cow-calf enterprses account for the hghest GHG emsson n the beef producton [11], whch s msleadng as most of these analyses do not consder the GHG sequestraton n the ecosystem beng studed. In addton, these analyses do not consder the GHG emssons generated by croppng practces [45,49] and sol eroson [50] assocated wth grans fed durng the non-grazng portons of the producton cycle. Net GHG emssons dffer when the C sequestraton under the dfferent grazng practces s consdered whch underlnes the mportance of takng nto account both GHG emsson and C sequestraton smultaneously n such analyses. 4. Conclusons Usng the LCA approach, ths paper calculated GHG emssons of the cow-calf farms n the SGP regon. Results show that the overall GHG emssons and man GHG emsson sources of the SGP regon dffer from those of the rest of the U.S. such as the NGP regon, and other countres n the world. Ths ndcates the mportance of the LCA analyss on a regonal bass. In SGP, where accordng to our fndngs, overall GHG emssons are hgher than the other regons, almost 80% of GHG emssons are from enterc methane. There s great potental of reducng GHG emsson by ncreasng grass qualty and dgestblty, whch could reduce total GHG emssons by as much as 30%. Compared to contnuous grazng, MP grazng can mprove grass qualty [38] as well as grass producton [15], thus, MP grazng strategy s potentally a good opton to reduce GHG emsson on a cow-calf farm. Unlke most publshed work that solates the analyses of GHG emsson and C sequestraton, our paper used feld observed SOC data to estmate the C sequestratons for dfferent grazng management systems. Contrary to the publcatons clamng that cow-calf farms are the most sgnfcant GHG emsson source n the beef producton lnk, our results show that cow-calf farms convertng from HC to MP or LC practces n SGP regon are lkely net carbon snks. In our study, the hghest SOC stock occurred upon convertng to MP grazng ndcates that among the three dfferent grazng practces we analyzed, MP has the hghest carbon sequestraton rate. Combned wth ts potental to sgnfcantly lower GHG emssons, we conclude that MP serves as the best carbon mtgaton opton.